Golf balls traditionally have been categorized in different groups, namely as one-piece, two-piece and three-piece balls. One-piece balls comprise a solid molded mass of the same material and do not have a core. Two-piece golf balls comprise a solid resilient core and an outer cover comprised of a different type of material molded thereon. Three-piece golf balls traditionally include a liquid or solid core, an elastomeric winding around the core and an outer cover molded thereon.
The outer cover of either two or three-piece golf balls may comprise single or multiple layers of molded material. The cover material may be balata (transpolyisoprene, natural or synthetic rubbers), although synthetic covers comprising non-ionomeric materials, for example polyurethane, or ionomeric polymers (polymers containing interchain ionic bonding) have become increasingly prevalent.
Additionally, despite their two and three-piece names, golf balls of either type may also comprise additional layers intermediate the cover and core or windings. The intermediate layers may be comprised of a wide range of materials, including polymers such as polyurethane, non-ionomeric and ionomeric materials.
Typically, the layers of multilayer golf balls are formed by applying them around the golf ball core or a preceding intermediate layer. Conventional techniques for applying such layers include injection molding, compression molding and casting the layer material around the underlying structure. A crucial aspect of the manufacture of multilayer balls is obtaining good adhesion strength between the various layers. If the adhesion strength between the layers does not meet desired levels, the performance of the golf ball will be adversely affected. For example, poor adhesion can cause air pockets between the layers, which can result in separation of the layers when the golf ball is struck with a club.
Further, as can be seen from the above, the possible combinations of core layers and materials, intermediate layers and materials and cover layers and materials are very large. As would be expected, the materials of one layer are often dissimilar in physical and chemical properties from those of an adjacent layer. This can make achieving a desired adhesion strength between the dissimilar layers difficult. In any configuration however, the material of each of the golf ball layers must be securely joined to the adjacent underlying and overlying layers to provide acceptable adhesion strength, performance and durability.